When President Clinton was impeached, it challenged the American public and the American political system to think about what constituted an impeachable offense and what did not.
The Constitution does not tell us in any detail what an official can be impeached for. It simply mentions "high crimes or misdemeanors." But what does that mean? That was the issue that faced the US.
President Clinton was impeached for lying under oath. This is, of course, a serious offense. However, he lied under oath about a personal matter (his sexual history) that had nothing to do with his official duties.
The question then was whether lying under oath about a personal matter was an offense that was grave enough to warrant impeachment. This was one of the main issues presented by the case. The other, related, issue was that of the degree to which partisan politics ought to play a part in the impeachment process. Should a president be impeached for an action that was somewhat serious (much less serious than that of Richard Nixon, for example) if the major motive for bringing the impeachment was political?